Coating processes – With pretreatment of the base
Reexamination Certificate
1997-09-29
2003-01-07
Utech, Benjamin L. (Department: 1765)
Coating processes
With pretreatment of the base
C427S319000
Reexamination Certificate
active
06503565
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for treating metal surfaces and a treating solution for use in such a process. The invention also relates to a metal surface treated by the process of the invention. The process is particularly useful for cleaning metal surfaces, such as in a pretreatment of metal surfaces. In such a pretreatment application, the process may provide a uniform and chemically active surface prior to further surface treatment, such as the application of a coating by painting, conversion coating, anodising or plating.
BACKGROUND OF THE INVENTION
In technologies dealing with pretreatment of metal surfaces, a clean uniform metal surface is often crucial in the overall effectiveness of the treatment process. In particular, a uniform, chemically active metal surface is very important for the adherence of an applied coating such as paint, powder coatings, polymer coatings and conversion coatings.
While surface impurities and/or contamination can be successfully removed by mechanical abrasion of the metal, mechanical abrasion is labor intensive and therefore uneconomical. It may also lead to excessive pitting and other damage to the surface. Chemical cleaning is therefore generally favoured.
One common means of chemically cleaning metal surfaces is by treatment with alkaline based solutions. Such solutions dissolve contaminants and impurities such as oxides from the surface of the metal, but may also etch surface oxides and/or metal. The result is often that a smut is left on the surface of the metal which requires further treatment of the metal to remove it. As used herein, the term “smut” is intended to include impurities, oxides and any loosely-bound intermetallic particles which as a result of the alkaline treatment are no longer incorporated into the matrix of the alloy.
Traditionally, removal of smut left after alkaline treatment has been effected by acidic solutions having effective amounts of appropriate additives. These “de-smutting”, or “deoxidising”, solutions remove smut from the metal surface and preferably etch the metal surface to remove oxide scale in order to leave a substantially homogeneous surface for any subsequent treatment. Many such prior desmutting solutions contain chromium ions. The use of chromium-containing desmutting solutions is particularly prevalent, but not restricted to, the field of metal conversion coatings. The term “conversion coating” is a well known term of the art and refers to the replacement of native oxide on the surface of a metal by a controlled chemical formation of a chemical film. Oxides or phosphates are common conversion coatings. Conversion coatings are used on metals, such as aluminium, steel, zinc, cadmium or magnesium and their alloys, and provide a key for paint adhesion and/or corrosion protection of the substrate metal. Accordingly, conversion coatings find application in such areas as the aerospace, architectural and building industries.
In recent years however it has been recognised that the hexavalent chromium ion, Cr
6+
, is a serious environmental and health hazard. Consequently, strict restrictions have been placed on the quantity of Cr
6+
used in a number of industrial processes and limitations placed on its release to the environment, leading to costly effluent processing.
There is clearly a need for an alternative metal treating solution which effectively cleans metal surfaces but does not pose the same environmental and health risks of the prior art.
An object of the present invention is therefore to overcome, or at least alleviate, one or more of the difficulties and/or deficiencies related to the prior art.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a process for cleaning a metal surface including the steps of:
(a) contacting said metal surface with an alkaline cleaning solution in order to remove contaminants such as dirt and grease; and
(b) contacting said metal surface with an acidic, rare earth ion containing solution thereby to remove smut formed on said metal surface by step (a).
The present invention also provides an acidic, rare earth ion containing aqueous cleaning solution for use in step (b) of the process defined in the preceding paragraph, said solution including ions of one or more rare earth ions, wherein the pH and concentration of rare earth ions in solution are effective to remove smut from a metal surface previously contacted with an alkaline cleaning solution.
Steps (a) and (b) of the treating process of the present invention may be used as a pretreatment of a metal surface prior to a subsequent finishing treatment such as applying paint or a coating. It is particularly useful as a pretreatment of metal surfaces prior to the application of a conversion coating thereto, such as a rare earth element based conversion coating.
One such conversion coating process has been described in Australian patent specification AU-A-14858/88. The conversion coating process comprises contacting a metal surface with a solution formed by an aqueous acidic solution containing cerium cations and H
2
O
2
in which some or all of the cerium cations have been oxidised to the +4 valence state. Gaseous evolution in the region of the metal surface causes an increase of the solution pH to a sufficiently high value to precipitate a cerium containing coating on the metal surface.
Accordingly the present invention further provides a process for forming a rare earth element containing coating on the surface of a metal, including the steps of:
(a) contacting said metal surface with an alkaline cleaning solution to remove surface contaminants such as dirt, grease and oxides;
(b) contacting said metal surface with an acidic, rare earth ion containing cleaning solution thereby to remove smut formed on said metal surface during step (a); and
(c) contacting the metal surface with an aqueous acidic, rare earth ion containing coating solution including rare earth cations capable of having more than one valence state, resulting in an increase of the pH of the acidic solution in the region of the metal surface to a value sufficient to precipitate one or more compounds of the rare earth element, thereby to cause the compound of the rare earth element to precipitate in a coating on the metal surface.
Pretreatment of the metal surface by steps (a) and (b) of the present invention is found to result in improved corrosion resistance and/or at least similar adhesion characteristics of the subsequently applied coating compared to the properties of a rare earth element based coating applied to a metal surface which was not subjected to any pretreatment or was instead pretreated with a chromate based cleaning solution. Also, the rare earth pretreatment results in a shorter time being subsequently required to deposit the rare earth element-based coating, as compared to other metal pretreatments, such as Cr based deoxidising solutions. Moreover, the absence of Cr
6+
in the solutions used significantly reduces the risk to health and the environment.
The step of contacting with an alkaline cleaning solution may be preceded by a degreasing step in which the metal surface is contacted with a degreasing composition, such as trichloroethane or a solution available under the trade name of BRULIN, which is an aqueous degreasing solution. A degreasing step may be necessary, for example, where the metal has been previously coated with lanoline or other oils or grease or with a plastic coating.
The alkaline cleaning solution is preferably a “non-etch” solution, that is, one for which the rate of etching of material from the metal surface is slow. A suitable alkaline cleaning solution is that commercially available under the trade name RIDOLINE 53.
The treatment with an alkaline cleaning solution is preferably conducted at an elevated temperature, such as up to 80° C., preferably up to 70° C.
Preferably the metal surface is rinsed with water between each of the above steps (a) to (c).
Treatment with the acidic, rare earth ion containing cleaning solution of step (b) is des
Henderson Mark Julian
Hinton Bruce Roy William
Hughes Anthony Ewart
Nelson Karen Joy Hammon
Nugent Sally Ann
Commonwealth Scientific and Industrial Research Organisation
McDermott & Will & Emery
Utech Benjamin L.
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